1. Field of the Invention
The invention relates to glass compositions, and more particularly to glass compositions for gaskets, seals of apparatuses operating at high temperatures, for example from 600 to 1000° C. notably from 700 to 900° C.
More specifically, the invention relates to glass compositions for gaskets, seals of a high temperature electrolyzer (<<HTE>>) or of a high temperature fuel cell (Solid Oxide Fuel Cell or SOFC) comprising a stack of elementary cells.
The invention further relates to a method for assembling at least two parts by applying said glass compositions. These parts are notably parts which enter the structure of a high temperature electrolyzer or of a high temperature fuel cell (Solid Oxide Fuel Cell or SOFC).
The technical field of the invention may thus be generally defined as that of glass gaskets, the function of which is to ensure the seal between the different compartments of apparatuses in which fluids are conveyed at high temperatures. More particularly, the technical field of the invention is that of glass gaskets ensuring the seal between the different compartments in which gases are transported and produced in high temperature electrolyzers or high temperature fuel cells, notably those comprising a stack of elementary cells which generally operate between 600 and 1,000° C., in particular between 700° C. and 900° C.
2. Description of the Related Art
In high temperature electrolyzers, electrolysis of water at a high temperature is performed from vaporized water. The function of a high temperature electrolyzer is to transform steam into hydrogen and oxygen according to the following reaction: 2H2O(g)→2H2+O2.
This reaction is conducted via an electrochemical route in the cells of the electrolyzer.
Each elementary cell, as shown in FIG. 1, consists of two electrodes, i.e. an anode (1) and a cathode (2), sandwiching a solid electrolyte generally in the form of a membrane (3).
Both electrodes (1, 2) are electron conductors and the electrolyte (3) is an ion conductor.
The electrochemical reactions occur at the interface between each of the electron conductors and the ion conductor.
At the cathode (2), the half reaction is the following: 2H2O+4 e−→2H2+2O2−; 
And at the anode (1) the half reaction is the following: 2O2−→O2+4e−.
The electrolyte (3) placed between both electrodes is the migration location of the O2− ions (4) under the effect of the electric field generated by the potential difference imposed between the anode (1) and the cathode (2).
An elementary reactor, illustrated in FIG. 2, consists of an elementary cell (5) as described above, with an anode (1), an electrolyte (3), and a cathode (2) and of two monopolar connectors or more exactly two half-interconnectors (6, 7) which ensure electrical, hydraulic and thermal functions. This elementary reactor is called a module.
In order to increase the produced hydrogen and oxygen flow rates, and as this is shown in FIG. 3, several elementary modules are stacked (8), the cells (5) then being separated by interconnectors or bipolar interconnection plates (9).
The assembly of the modules (8) is positioned between two upper (10) and lower (11) interconnection plates which bear electric power supplies and gas supplies (12). This is then referred to as a stack (FIG. 3).
There exist two designs, configurations, architectures for the stacks:                tubular stacks, in which the cells are tubes, and        planar stacks, in which the cells are made in the form of plates like in FIG. 3.        
In the planar architecture, the cells and the interconnectors are in contact in many points. The manufacturing of the stack is subject to fine tolerances as to the flatness of the cells in order to avoid too high contact pressures and inhomogeneous distribution of the stresses, which may lead to cracking of the cells.
The seal gaskets in a stack have the purposes of preventing a hydrogen leak from the cathode to the neighboring anodes, of preventing an oxygen leak from the anode towards the neighboring cathodes, of preventing a hydrogen leak towards the outside of the stack and finally of limiting the steam leaks from the cathodes towards the anodes.
Within the scope of stack development for high temperature electrolysis (<<THE>>), and as this is shown in FIG. 4, gas-proof gaskets (13) are thereby made between the planar electrolysis cells (5), each consisting of an anode/electrolyte/cathode ceramic trilayer, and the metal interconnectors or interconnection plates (9).
It should be noted that the dimensions given in μm in FIG. 4 are only given as examples.
More specifically, a gasket is made between the lower surface of each cell (5) and the upper half-interconnector (14) of the interconnection plate located below the cell on the one hand, and between the upper surface of each cell and the lower half-interconnector (15) of the interconnection plate located above the cell (5) on the other hand.
These gaskets (13) generally have to have a leak rate in air of less than 10−3 NmL/min/mm between 700° C. and 900° C. under a pressure difference from 20 to 500 mbars.
In addition to this seal function, the gasket may in certain cases have secondary assembling and electric conduction functions. For certain stack architectures, a ceramic part, called a cell support, may be placed between the cells and the interconnectors; and gas-proof gaskets are then also required with this cell supporting part.
Several seal solutions are presently investigated, i.e.: cements or ceramic adhesives, glasses or vitroceramics gaskets, metal compressive gaskets, mica compressive gaskets, brazed gaskets and mixed solutions resorting to several of these techniques.
These gaskets should give the possibility of ensuring the seals between the cathode chamber and the outside, between the anode chamber and the outside, and between both chambers, and thereby avoiding gas leaks between both chambers and towards the outside.
As this has already been specified above, we are most particularly interested in glass gaskets herein.
The glasses used for these gaskets may either be made of a simple glass, or of a crystallizable glass also called a vitroceramic, or further a mixture of both of these glasses, or further a simple glass to which are added ceramic particles.
Most glasses used for these gaskets are generally in solid form at the temperature of use i.e. generally between 600° C. and a 1,000° C., notably between 700° C. and 900° C., for example 850° C. These gaskets are described as <<hard>> gaskets and generally have a viscosity of more than 109 Pa·s at 850° C.
The main constraint to be observed in this situation is to formulate a gasket having a thermal expansion coefficient <<TEC>>, adapted to the other elements of the junction, notably to the ceramic and metal parts.
As regards simple glasses, SiO2—CaO—B2O3—Al2O3 compositions are studied in document [1], BaO—Al2O3—SiO2 compositions are described in document [2] and in document [3], and finally LiO2—Al2O3—SiO2 compositions are mentioned in document [4], but with these compositions it is difficult to attain TECs adapted to the junctions.
Vitroceramic glasses are as for them generally shown as being more chemically and mechanically resistant thanks to the control of the crystallization of the glass by means of nucleating agents and particular heat treatments.
The parameters to be controlled for these vitroceramic glasses are the formulation of the glass and the heat cycles in order to manage formation of the crystalline phase(s) having the sought properties.
Thus, vitroceramic glass compositions of the LAS (LiO2—Al2O3—SiO2) type are described in document [4], compositions of the BAS (BaO—Al2O3—SiO2) type are studied in documents [2] and [6], compositions of the BCAS (Barium Calcium AluminoSilicate) type are mentioned in documents [7] and [8], and finally SiO2—CaO—MgO—Al2O3 compositions are the subject of document [9].
However, development of the formulations and of the heat treatments for vitroceramic glasses remains delicate since the junction material changes over time, with modification of the crystalline phases and because of the creation of interfaces between the materials in contact. Industrial development of this type of vitroceramic glasses therefore remains complex.
Finally, by adding ceramic particles of different sizes and shapes to simple glasses it is possible to control and adjust the viscosity and the TEC of the sealing material [10, 11]. The delicate point lies in the presence of a glassy phase in a large amount which may pose high temperature corrosion or evaporation problems.
In addition to the <<hard>> gaskets described above which appear in solid form at the operating temperature, SrO—La2O3—Al2O3—B2O3—SiO2 compositions with which a fluid condition of the glass may be obtained at the operating temperatures, are disclosed in document [5]. With these compositions, it is possible to accommodate the large TEC differences, but the formulations developed in this document do not seem to be sufficiently resistant from a mechanical point of view, exactly because of this too large fluidity of the glass, in order to be able to maintain the seal against the imposed pressure differences.
Document [12] describes a sodium-sulfur cell which comprises a solid electrolyte tube, an insulating ring which electrically insulates a positive electrode compartment from a negative electrode compartment, and a gap from 100 to 500 μm between the solid electrolyte tube and the insulating ring and a glass solder which fills this gap in order to attach the insulating ring to the electrolyte tube.
In order to achieve the assembling between the solid electrolyte tube and the insulating ring, the lower portion of the electrolyte tube is inserted into the insulating ring, a glass ring is inserted into the gap formed between the solid electrolyte tube and the insulating ring, this is then heated and the glass ring is melted in an electric oven.
The brazing glass is an alumino-borosilicate glass, for example comprising the 4 following ingredients in % by weight:                0 to 80% by weight of SiO2;        0 to 30% by weight of Al2O3;        0 to 80% by weight of B2O3;        and 0 to 30% by weight of Na2O.        
Examples of brazing glasses SiO2/Al2O3/B2O3/Na2O are given in Table 1 of document [12]. It should be noted that the compositions of the glasses of Table 1 are expressed in % by weight.
Further, it is noted that the composition E of Table 1 is not normalized to 100 and consequently any comparison with this composition of document [12] is impossible.
The claimed composition (A) differs from compositions of this document, in particular as regards the B2O3 content.
Further, the glasses described in U.S. Pat. No. 5,196,277 [12] are welding glasses for low temperature sealing applications, unlike the claimed compositions (A) and (B) which are specifically formulated for high temperature sealing applications and which have properties, in particular viscosity properties but also low reactivity properties towards the materials in contact, suitable for this application.
It emerges from the foregoing that presently there does not exist any glass composition which is satisfactory for a use in seal gaskets for apparatuses operating at high temperature such as high temperature electrolyzers or high temperature fuel cells.
Therefore, there exists a need for a glass composition which gives a chemically and mechanically resistant gasket, notably having mechanical properties allowing it to be adapted to the occasionally very different TECs of the materials to be assembled.
There also exists a need for a glass composition which is not subject to high temperature corrosion or evaporation phenomena.
There is further a need for such a glass composition which has no or few interactions with the materials to be assembled.
Additionally, there exists a need for a glass composition which may be prepared reliably, easily and reproducibly without resorting notably to complex heat cycles.
Finally there exists a need for such a glass composition, all the properties of which remain stable over time in particular under high temperature conditions.